Turbines

Today turbines are widely used because they create mechanical drive and generate power simultaneously. Turbines work by converting the kinetic and thermal energy of a moving fluid, usually gas, water, air or steam, into mechanical energy.

(Copyright Capicitec, Inc., Ayer, MA)

Gas

Gas turbines combust liquid or gaseous fossil fuels in the presence of compressed air and then convert the thermal and kinetic energy of the product gases into mechanical energy.

(Copyright Alstom Power, Inc., France)

General Information

Gas turbines were invented around the turn of the 19th century to turn a shaft and create mechanical energy that could power a generator . The first jet aircraft engine using a gas turbine was built in 1939. By 1950, gas turbines had became common for planes, boats, and generators. The picture below shows the shaft of a gas turbine with various sizes of blades attached.

(Copyright Alstom Power, Inc., France)

There are two types of gas turbines: aero-derivative gas turbines and heavy-industrial gas turbines. Aero-derivative gas turbines are lighter, reach higher speeds, and are much easier to maintain than heavy-industrial turbines. Because of these properties, aero-derivative gas turbines are most often seen in the aircraft and space industries. They are also used in remote locations, such as offshore drilling rigs, due to their high reliability and low maintenance requirements.

Heavy-industrial turbines move at slower speeds and have significantly larger volumes of air flow relative to aero-derivative turbines. Therefore, heavy-industrial turbines also consume more fuel and use approximately 50% more air. Additionally, heavy-industrial turbines are difficult to service. They are, however, much more resistant to corrosion because of their thick blades.

Gas turbines work using the Brayton cycle , which consists of adiabatic compression, constant pressure heating, and adiabatic expansion over a turbine, generating power.

Equipment Design

Gas turbines are made up of three units; a compressor, a combustion chamber, and a turbine or rotor. The compressor increases the pressure of the input air adiabatically. For more information see the
Compressors
section of the encyclopedia. The compressed air passes to the combustion chamber, where it is mixed with fuel and burned. Combustion chambers can be tubular or annular. Tubular combustion chambers have a cylindrical lining concentric with a cylindrical casing. Annular combustion chambers have an annular lining and annular casing. Combustion chambers typically receive a 50:1 to 150:1 air to fuel volume ratio. The fuels could be liquid, such as diesel fuel, or gas, such as natural gas.

The combustion produces hot gases that rush through the turbine blades held on a rotor. The kinetic energy of the moving gases is converted to mechanical energy as it rotates the turbine blades. If it is an industrial gas turbine, the expansion of the gases over the turbine rotates a shaft. The shaft performs work on a load to produce the electricity in the generator. The shaft often performs work to run the compressor as well. The exhaust gases are then vented or in some cases are reheated and passed over additional turbines.

Shown below is an exploding diagram of a gas turbine. Below that is a picture of an actual shaft and turbine blades.

(Copyright NASA, Washington, DC)

(Copyright Alstom Power, Inc., France)

Gas turbines can be set up in either a cold-end drive or a hot-end drive configuration. A hot-end drive configuration is more common. In this configuration the output shaft is located where the hot exhaust gases exit. This makes the turbine more difficult to service, decreases the operational lifetime of its bearings and can result in excessive vibration or power loss. In cold-end drive configurations the output shaft connects to the front of the air compressor. This makes for easier maintenance, but has one disadvantage. The shaft connected on the front of the air compressor can create turbulence at the inlet duct. If turbulence occurs at the inlet, it could induce a surge through the system that would destroy the entire turbine in seconds.

Turbine blades are typically made of metal, often stainless steel or titanium alloys. They can come in various arrangements, sizes, and amounts depending on the application.

Gas turbines have two types of turbine blades, shown in the animation. Stators are stationary, and are intended to increase the gas velocity, transform thermal energy into kinetic energy, and redirect the gas flow into the direction of rotation of the second set of blades. The second type of blades, rotors, are rotated by the moving gas, transforming kinetic energy to mechanical energy. The mechanical energy created is then used to power the compressor and an external load.

Usage Examples

Gas turbines are typically used as aircraft engines. The plane pictured below combusts jet fuel to rotate a turbine that ejects high-velocity gas, propelling the plane. In addition, jet propulsion, or reaction propulsion, can be used to propel watercraft.

(Copyright NASA, Washington, DC)

Gas turbines are also used to generate electricity. The generator pictured below uses a gas turbine to turn an electrical generator. Gas turbines are often used in conjunction with steam turbines for power plants with ratings up to around 3000 megawatts.

(Copyright Tenaska 6-3-2011, Omaha, NE)

Advantages

Disadvantages

High reliability.

Low lubricating oil consumption.

Can be built for power plants rating 2000 MW and higher.

Gas turbines can reach full power very quickly, less than two minutes for small generators.

Mechanical energy can be easily transformed with the addition of secondary equipment.

Low maximum efficiencies.

Require high-grade fossil fuels to combust.

Produce exhaust gases, pollution concerns.

Must be precisely built to avoid low efficiency.

Temperatures must be kept high.

Susceptible to corrosion.

Temperature and humidity can have a large effect on power output.

Noisy.

Steam

Steam turbines transform the thermal and kinetic energy of steam into mechanical energy.

General Information

The first steam turbine, an impulsive-type steam turbine, was developed by Carl Gustav de Laval in 1883. In 1884, Charles Parsons altered the design to create the reaction steam turbine.

Steam turbines follow the Rankine cycle . Steam has a low density compared to liquid water, which allows the velocity of steam to be up to 100 times greater than water. This high-velocity steam rotates turbine blades to produce mechanical energy.

Equipment Design

Steam turbines work in a way that is similar to gas turbines, but instead of using the hot product gases from a combustion reaction, steam is used. The hot steam is passed through stationary nozzles that transform the thermal energy of the steam into kinetic energy. The high velocity gas is passed over the turbine blades. By rotating the turbine blades, the kinetic energy of the steam is transformed into mechanical energy. This mechanical energy is used to rotate a shaft, which performs work on a load in addition to operating the compressor. Shown below to the left is a picture of a steam turbine and to the right are examples of different sized turbine blades.

Steam turbines come in two forms, reaction and impulse.

Reaction turbines have up to 40 sets of stationary and rotor blades . Stationary, or stator, blades redirect the steam and convert thermal energy into kinetic energy, accelerating the steam onto the rotor blades. The rotor blades are free to rotate. They are propelled by the kinetic energy of the steam. The pressure drop across the two sets of blades force the rotors to turn.

Impulse turbines have only rotor blades, resulting in little pressure drop. The energy of the steam is transferred to the turbine only by the steam jets striking the blades and rotating the turbine.

Usage Examples

Turbines are designed to turn a shaft, which operates a load. The load that a steam turbine operates most often is an electrical generator. The Beaver Valley Power Station, a nuclear power plant shown below, uses a steam-driven turbine to generate electricity. For more information see the
Nuclear Reactors
section of the encyclopedia.

(Copyright U.S. Nuclear Regulatory Commission)

In addition, steam turbines are used to drive centrifugal pumps, blowers, compressors and ship propellers. Steam turbine-powered electrical generators can have capacities exceeding a million kilowatts. In 1994, 85 percent of the capacity of all U.S. power plants was based on steam turbines.

Advantages

Disadvantages

Can be built to produce large quantities of mechanical energy, compared to gas turbines.

Mechanical energy can be easily transformed into electrical energy with the addition of secondary equipment.

Steam velocity can be 100 times greater than water velocity, making steam turbines more efficient than hydraulic turbines.

Can power other equipment, such as its own compressor.

Require the combustion of fossil fuels or fission to heat water.

Temperatures must be kept high for high efficiency.

Hydraulic

Hydraulic turbines use energy from falling water to turn a rotor, transforming kinetic energy to mechanical energy that is used to turn a shaft.

(Copyright United States Department of the Interior,

Bureau of Reclamation - Lower Colorado Region)

General Information

Hydraulic turbines use the kinetic energy of falling water to cause rotation. This rotation is transferred into mechanical energy as the turbine rotates a horizontal or vertical shaft. This shaft is most often used to drive electric generators to create electricity.

The type of turbine used and the amount of kinetic energy of the falling water is directly proportional to the distance the water falls, called the head . Below is an example of a Kaplan turbine, showing their immense size.

(Copyright Oak Ridge National Laboratory, Oak Ridge, TN)

Equipment Design

There are three kinds of hydraulic turbines: Pelton, Francis, and Kaplan. Water must be fed to all three at a high velocity. This is achieved by allowing the water to fall from a reservoir, known as the headwater, through a penstock, or pressure conduit. The water then passes over a turbine, which drives a shaft connected to a load, usually an electrical generator.

Once the water has passed out of the turbine, the water is discharged through a draft tube to the tailwater or tailrace conduit. The height between the headwater and the tail water is the static head.

The amount of water that is released from the headwater and that passes over the turbine can be controlled, allowing control over the rotation rate of the turbine and therefore the rate of electricity production.

Pelton turbines are the only type of impulse hydraulic turbine in use today. Impulse turbines contain only rotor, or movable, blades. They rely strictly on the impact of the water to rotate the runner. Impulse hydraulic turbines use jet nozzles to increase the velocity of water. The water exits the nozzles and impacts the buckets on the rim of a wheel. Buckets can be fixed with bolts, studs, or clamping rings, or the entire wheel can be cast as one piece, making the buckets nondetachable. Shown below to the left is an animation demonstrating the process of a typical Pelton turbine and to the right is an example of the buckets.

(Copyright Oak Ridge National Laboratory, Oak Ridge, TN)

Pelton turbines require heads of at least 800 feet, but can achieve efficiencies of 90%. These turbines can have one to six jets of water driving the runner, depending on the design of the turbine. Increasing the number of jets can increase the power of the turbine. Pelton turbines can be oriented horizontally or vertically. The Pelton turbine shown below is an example of a horizontally oriented shaft with four jets. The horizontal design is easier to maintain but is larger than the vertically oriented Pelton turbine.

(Copyright Oak Ridge National Laboratory, Oak Ridge, TN)

The Francis turbine , also called radial flow turbine, is a reaction-type turbine, that has both stator and rotor blades. Francis turbines are completely submerged in water received from the penstock after passing through a wicket gate. Wicket gates have 20 to 32 guide vanes arranged in a circular cascade. Wicket gates start the rotation of the water prior to reaching the runner. The vanes can be adjusted to increase efficiency. Guide vanes act as the nozzles, guiding the water tangentially and radially inward. As the water passes through it the turbine rotates, rotating the attached shaft. Francis turbines are best used with static heads of 80 to 1,000 feet, and they produce efficiencies of 90 to 95%.

(Copyright U.S. Geological Survey)

Pictured below is an example of a Francis turbine impeller that is used in hydroelectric power plants.

(Copyright Oak Ridge National Laboratory, Oak Ridge, TN)

Kaplan hydraulic turbines are also classified as reaction turbines. They are similar to Francis turbines, but have adjustable propeller blades, allowing maximum efficiency over a range of heads. Pivots at the base of the blades allow the angle of the blades to be changed during operation. The automatic adjusting propeller blades combined with the automatic adjusting of the wicket gates allow the flow to be controlled and therefore allow the Kaplan turbine to be used over a much wider range of applications than other turbines. The biggest advantage of Kaplan turbines is their ability to work with low-head applications. There are between four and eight blades on each runner, depending on the head. Generally the greater the head, the more blades are required. The animation shows a typical Kaplan turbine. Water from the reservoir passes through the penstock, through a set of stator blades, often a wicket gate, then through the rotor blades. The water causes the rotation of the blades and an attached shaft. Kaplan turbines are best used for lower head situations, often 3 to 330 feet.

Usage Examples

Hydraulic turbines are used almost exclusively for the production of electricity. The Hoover dam, pictured here, blocks the Colorado river to form Lake Mead. The dam contains a hydroelectric power plant that uses the static head difference between the reservoir and the Colorado river. The reservoir can hold over 325,000 gallons of water and the hydraulic turbines generate four billion kilowatt-hours of electricity annually.

(Copyright United States Department of the Interior,

Bureau of Reclamation - Lower Colorado Region)

Pictured are a shaft, left, and some generators, right, inside the Hoover Dam hydroelectric power plant.

(Copyright United States Department of the Interior, Bureau of Reclamation - Lower Colorado Region)

Another example of a hydraulic turbine is shown below. This Energy Recovery Turbine (ERT) is used in desalination plants to recover hydraulic energy that remains in the brine stream after the reverse osmosis process. Please see the
Membranes
section of the encyclopedia for more information on the reverse osmosis process. The brine stream rotates the ERT rotor and uses that energy to run the high pressure pump earlier in the process. In this way the turbine can utilize the energy in the desalination process more efficiently.

(Copyright Flowserve Corporation, Irving, TX)

Advantages

Disadvantages

Efficiencies of 95% can be reached.

Fluid does not need to be heated.

No combustion or fission required, as in gas and steam turbines.

Adjustable Kaplan blades maximize efficiency.

Can be modified to pump water.

Reservoir can serve as an irrigation source.

Turbine blades are large and complicated to engineer.

Can only be used where water will encounter a large drop, usually requiring a dam.

Water reservoir necessary.

Wind

General Information

Wind turbines convert the kinetic energy of wind into mechanical energy usually for the purpose of generating electricity. Generating large amounts of electricity from wind turbines requires a group of wind turbines, typically called a wind farm. Wind mills, simple wind turbines, have been in operation for well over 2,000 years. They were designed not to generate electricity, but with the same idea of turning kinetic energy into mechanical energy.

Wind turbines are impulse-type turbines because they contain only rotor blades. Wind turbines have been built with between two and 40 blades, but typically are limited to two or three blades, which are between 5 and 20 meters long. The most common blade materials are glass fiber, carbon fiber, and Kevlar reinforced plastics. The rotor turns a shaft, which enters a nacelle. The nacelle is the area behind the blades which contains the power generation equipment. The nacelle and the rotor are held aloft by a tower that is typically 20 to 40 meters tall.

Equipment Design

Nacelles, such as the one in the animation, contain all the power generation equipment. The shaft, which is turned by the blades, enters the nacelle and is connected to a generator. The generator creates electricity, which travels down the tower by cable to a transformer. A yaw mechanism keeps the blades perpendicular to the wind, maximizing efficiency. A yaw rotation, caused by the wind, is an off-axis rotation that shifts the blades. Hydraulic brakes are used to control the blade speed.

The picture below is an example of a wind turbine used for research by NASA. The turbine has lightweight carbon-fiber blades that are 33 feet in diameter. This turbine was tested in the world's largest wind tunnel in April 2000.

(Copyright NASA, Washington, DC)

Advantages

Disadvantages

Non-polluting source of energy.

Wind is abundant and free.

Hydraulic breaks and yaw mechanism allow great control over efficiency.